Conventional automobiles which use fossil fuels for the primary driving energy are exerting adverse effects on the earth's environment by emitting exhaust gases. In contrast, a wider use of electric cars which do not emit exhaust gases at all or hybrid cars which emit them in considerably reduced amounts holds much promise for improving the environment and is recommended from the viewpoint of effective utilization of surplus electricity generated at night. It is also desired to provide distributed power storage systems for stabilizing the supply of electrical power.
The conventional storage batteries, however, have involved serious problems such as extremely heavy weight or high cost. For example, aqueous solution based batteries such as lead-acid batteries which feature low unit price per energy are extremely heavy whereas organic solution based batteries such as lithium-ion batteries which feature high energy density and light weight are very expensive.
Organic solution based batteries such as lithium-ion batteries are less tolerant of overcharge and overdischarge and use flammable electrolytes, so if overcharged or overdischarged, they may potentially explode and suffer other danger on account of heat generation; hence, unlike aqueous solution based batteries such as lead-acid batteries, organic solution based batteries such as lithium-ion batteries usually need be individually provided with protective circuits and switches for protecting them against overcharge and/or overdischarge (see, for example, Patent Literatures 1 and 3).
According to the disclosure of Patent Literature 1, for example, lithium-ion batteries are mainly used for an assembled battery and cells of a lithium-ion battery are simply connected in series to make up an assembled battery or, alternatively, a plurality of cell groups each consisting of two or more cells connected in parallel are connected in series; in the former case, a protective circuit (anti-overdischarge circuit and/or anti-overcharge circuit) is provided for each cell, and in the latter case, the same protective circuit is provided for each cell group; in this way, capacity variations among individual cells or cell groups are reduced to be within an allowable range.
As a consequence, lithium-ion batteries are not only expensive on their own due to the high cost of the electrode materials; in addition, the overall cost is increased, largely by the electronic circuitry and components in the protective circuits and switches.
As disclosed in Patent Literature 1, the “protective circuits and switches” refers to protective circuits and switches that measure the cell or battery voltage and temperature as well as the current flowing through the cell or battery and which, when the measured value exceeds a specified level, causing the cell or battery to become exposed to an over-voltage, an over-temperature or an over-current, protects the cell or battery by cutting it off from the charging/discharging circuit.
Conventionally, such protective circuit and switches need not be used with lead-acid batteries but, as mentioned above, they are indispensable to lithium-ion batteries to avoid the risks of overcharge and overdischarge. When lithium-ion batteries are to be used on electric cars or power storage systems, their specified (rated) voltage and current are at least 10 times greater than the values specified for the ones that are used in mobile devices such as cell phones, laptop computers, and power tools and, as a consequence, the specifications of the protective circuits and switches need accordingly be adapted for high current and voltage but this requires provisions against heat, ohmic resistance, arc, etc. that considerably add to the overall cost.
To utilize the energy of fossil fuels effectively, it has been proposed that the energy generated from the deceleration of running vehicles such as automobiles but which is simply consumed as heat should be converted to electric energy as regenerative energy which is used to regenerate the current being supplied into a lead-acid battery (to charge it); in this case, for effective utilization of the regenerative energy, a lead-acid battery and a lithium-ion battery are connected in parallel to make a vehicular power supply system that is capable of accommodating a large regenerated current as produced from the braking such as deceleration of a running vehicle (see Patent Literatures 2 and 3).
According to Patent Literature 2, a lead-acid battery consisting of 18 series-connected cells and a lithium-ion battery consisting of 10-11 series-connected cells are connected in parallel to make a 42-V vehicular power supply system, in which in the process of charging by the braking such as deceleration of a running vehicle, the ratio of X/Y (where X is the value of a current flowing through the lead-acid battery incapable of accommodating a large current and Y is the value of a current flowing through the lithium-ion battery capable of accommodating a large current) is adjusted to lie between 0.05 and 1.00 so that the energy generated from the deceleration of running vehicles such as automobiles can be effectively utilized as regenerative energy.
According to Patent Literature 3, a lead-acid battery comprising a plurality of series-connected cells and a lithium-ion battery comprising a plurality of series-connected cells are connected in parallel to make a vehicular power supply system, wherein a safety circuit which, in the process of charging by the braking such as deceleration of a running vehicle, measures the voltage across each cell in the lithium-ion battery or the temperature of the lithium-ion battery and which, if the measured voltage value or the measured temperature exceeds a prescribed upper limit, finds the measurement abnormal and cuts off the charging or discharging of the lithium-ion battery, whereby the lithium-ion battery is protected from getting into an abnormal state to ensure the safety of the vehicular power supply system.